Field of Invention
The present invention generally relates to flexible tanks for liquids commonly known as collapsible tanks or pillow tanks. These are tanks made of a fluid resistant fabric that may be rolled or folded for transport, and when in use they may be unrolled and filled with liquid. The present invention relates to an overfill indicator for flexible collapsible tanks, and to a liquid fill level indicator for flexible collapsible tanks.
Description of Related Art
Flexible collapsible liquid tanks are used for the handling, storage and transportation of large volumes of liquids such as water or fuel. These tanks are commonly referred to as pillow tanks in the industry, and are typically made of a flexible fiber reinforced elastomeric material. They have inlets or nozzles for filling and draining purposes, as well as vents disposed on the top surface of the tank to allow the escape of air and other gasses during filling. A principal advantage to such flexible collapsible liquid containers is that they are relatively lightweight and compact when empty since they do not have a frame or rigid supports. The flexible and collapsible bladder is designed to contain the liquid; hence it is designed to be in constant contact with the liquid. The liquid may be a hydrocarbon fuel, in which case the material is made from a fabric that is compatible for use with hydrocarbon fuels. Such material is well known in the art of pillow tanks designed for hydrocarbon fuel storage. Examples include a heavy duty urethane coated fabric. Most large pillow tanks have one or two fill and drain ports to facilitate filling and discharging of liquid, as well as one or more vents (or flame arrestor vents in the case of fuel transport tanks) on the top wall of the tank. Some examples of flexible collapsible liquid tanks include those manufactured by SEI Industries Ltd. of Delta, Canada, and sold under the trademarks TERRA TANK™, ARCTIC KING™, DESERT KING™ and JUNGLE KING™.
Since collapsible pillow tanks do not have a fixed shape, it has caused challenges to the effective monitoring of the liquid volume inside. The hyper-elastic nature of the tank allows operators to easily overfill them without noticing the overfill condition. This can lead to stresses being put on seams in the tank, which over time can lead to tank failure.
There are methods and devices that have been used to date to monitor the amount of liquid inside large collapsible pillow tanks, but these have not worked well for various reasons. For example, one method involves metering the volume of liquid inflow into the tank and keeping records of the total volume of liquid put into the tank, attempting to ensure that the volume going into the tank is below the manufacturer's stated maximum capacity of the tank. However, this method often fails in the field due to poor record keeping of the amount of liquid going in and out of the tank.
Another method involves installing a pressure sensor on the bottom or floor of the tank for monitoring the hydrostatic head pressure of the liquid from which is calculated the volume of liquid in the tank. This method, while accurate, is expensive and the experience has been that the system for measuring tank volume in this way is rarely purchased by customers because of the costs.
Yet another method involves the use of an external measurement system comprising two sticks and a string across the highest point of the tank from which the height of the tank may be observed. This method is cumbersome for operators in the field and often results in poor readings because the distance between the two sticks is so great on large tanks and the string often sags.
Accordingly, there is a need for simple, cost effective and reliable overflow indicator devices or liquid level indicator devices for use on large flexible collapsible liquid tanks, also known as pillow tanks.